Let Backend determine support for deferred loading.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 92116b8f77d792fd51a4a55ad9c908c99ccb6a0d..145dabedc4a8f7c18cdfd0b9269c36d89550d851 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -770,16 +770,9 @@ class DeferredLoadTask extends CompilerTask {
         }
       });
     }
-    Backend backend = compiler.backend;
-    if (isProgramSplit && backend is JavaScriptBackend) {
-      backend.registerCheckDeferredIsLoaded(compiler.globalDependencies);
-    }
-    if (isProgramSplit && backend is DartBackend) {
-      // TODO(sigurdm): Implement deferred loading for dart2dart.
-      compiler.reportWarning(
-          lastDeferred,
-          MessageKind.DEFERRED_LIBRARY_DART_2_DART);
-      isProgramSplit = false;
+    if (isProgramSplit) {
+      isProgramSplit = compiler.backend.registerDeferredLoading(
+            lastDeferred, compiler.globalDependencies);
     }
   }